Fiber-metal-laminate structures are becoming an increasingly popular material choice where strength or weight, or both strength and weight, are taken into consideration. For example, fiber-metal-laminates are used in the aircraft and automotive industries as lightweight and high-strength alternatives to non-laminate structures.
Generally, a laminate structure having an outer laminate layer that is resistant to failure is preferred as an exterior layer. This is because the outer laminate layer protects the inner laminate layers and the bond layers from potentially harmful elements that may cause laminate structure failure, as well as providing an aesthetically pleasing surface. However, the generally thin nature of the laminate layers creates difficulties in maintaining the integrity of the outer laminate layer. Referring to FIGS. 1A and 1B, untreated outer metal laminate layers of fiber-metal-laminate structures are subject to residual tension stresses. Referring to FIG. 1A, before high temperature curing, an outer metal laminate layer 5 has an initial length l, and an inner laminate layer 7 has an initial length l2. Referring now to FIG. 1B, after cooling down from high temperature curing, the outer metal laminate layer 5 has a final length l3 that is longer than the initial length l1, and the inner laminate layer 7 has the final length l3 that is shorter than the initial length l2. As a result, the outer metal laminate layer 5 is subject to residual tension stresses that can contribute to the onset of crack initiation. For example, if the onset of crack initiation occurs at 40 KSI and the outer metal laminate layer 5 is subject to residual tension stresses of, for example, 20 KSI, then onset of crack initiation occurs at a lower stress level (in this example, at 20 KSI instead of 40 KSI) or at a reduced number of fatigue cycles at a given stress level.
Thus, fatigue life at an existing stress level is shortened when the outer metal laminate layer is subject to residual tension stresses. Alternately, for a finite fatigue life of a set number of fatigue cycles, a lower stress causes the onset of crack initiation as discussed above.
One current approach to enabling a fiber-metal-laminate structure to withstand greater stresses is to increase the amount of the structure. However, such an approach can become costly. Further, increasing the amount of structure adds weight to the structure and can offset weight advantages inherent in use of fiber-metal-laminate structures, especially in applications such as aerospace applications in which weight savings are advantageous.
Another current approach to enabling a fiber-metal-laminate structure to withstand greater stresses is to pre-stress the laminate structure. The entire laminate structure is generally subjected to a pre-stress that is designed to stretch the outer metal laminate layers past their yield point to reduce or eliminate residual tension stresses in order to delay the onset of crack initiation in the outer laminate layers. However, this process is employable only in sheet-type laminate structures and not with a laminate structure having a more complex geometry.
Therefore, there exists an unmet need in the art for decreasing the onset of crack initiation of fiber-metal-laminate materials of any geometry without adding structure or pre-stressing entire laminate structures.